The person skilled in the art is familiar with systems of TAWS type, “Terrain Awareness and Warning System”. The aim of these systems is to generate an alert when the aircraft is in a dangerous situation where the operational margins are no longer complied with. TAWSs in the guise of autonomous computer or computer integrated with the functions of TCAS, standing for “Traffic Collision Avoidance System”, and WXR standing for “Weather Band X Radar”, in an ISS, “Integrated Surveillance System”, fulfil a primary function of terrain anticollision monitoring (“Safety Net”) and are aimed at emitting audible alerts during an exceptional approach to the relief allowing the crew to react by engaging a vertical resource before it is too late. Accordingly, TAWS systems, decoupled from navigation systems, proceed in two ways. They periodically compare the theoretical trajectory that would be described by the aircraft during a resource and compare it with a section of the terrain and with the obstacles overflown obtained from a digital model of the world or local terrain embedded aboard the computer. Or else, certain TAWSs also integrate modes termed “reactive modes” which, by periodically comparing certain of the current parameters of the craft, for example the radio-altitude and the vertical speed, various charts determine whether the current situation of the aircraft is a normal situation or whether it is potentially dangerous. In the latter case, an alert, limited to a verbal message, is generated to inform the crew. The availability of a terrain model permits functions making it possible to improve the crew's situation perception. Among them, the objective of the alert lines is to delimit the terrain zones for which a TAWS alert might occur. For their part, the “Alert Areas” show the zones causing a TAWS alert. Numerous patent documents describe this type of system. Among them may be cited patent “EP0 565399B1” describing all the basic concepts of TAWSs and patent application “US2003/0107499A1” describing a device for displaying the terrain risk zones capable of causing a TAWS alert.
Helicopter laser radar systems (“HELLAS”) can fulfill a function of protecting against collisions between the craft and an obstacle, high-voltage lines for example, by preventing the aircraft from approaching too closely. They proceed by scanning the zone situated at the front of the craft by means of a laser beam invisible to the naked eye. Potential obstacles encountered are presented to the pilot via a cockpit display. An audio alert is optionally generated when the craft is considered to be too close to the obstacle.
Flight Control Systems can fulfil a function of protecting against collisions between the tail of the craft and the ground by preventing the aircraft from having an attitude, in terms of posture, such that the tail of the craft can enter with the ground. They proceed by comparing the attitude of the aircraft and the speed at which it is approaching the ground with the remaining height as well as with a predefined margin. If the various elements are not compatible, a corrective action is undertaken by the system on the controls of the craft.
The essential purpose of systems of TAWS type is to protect the craft during cruising flight. Cases of collision between the craft and an arbitrary obstruction usually occur during the takeoff or landing phases—phases during which the TAWS alert functions are disabled, or in the best of cases, with a sensitivity that is reduced so much that the alert will be delivered too late to save the craft and more particularly the anti-torque device situated on the tail of the helicopter. Certain systems of TAWS type integrate additional protection functions, such as for example a function for protecting the tail of the craft, but they take no account of obstacles situated laterally or to the rear of the craft. Only the relief situated under the craft is actually considered. Systems of TAWS type, through their use of a digital terrain model, are not capable of detecting mobile obstacles, such as terrestrial vehicles or other aircraft. The alert function of a TAWS system triggers a message destined for the crew as soon as a certain safety threshold is crossed. It does not make it possible to calculate or to picture the discrepancy with respect to this threshold.
The systems of “HELLAS” type are complex and expensive. Their purpose is essentially aimed at military use and they are generally intended for the detection of obstacles of high-voltage cable type in the cruising phase while flying at low or very low altitude. These systems do not generally make it possible to protect the lateral parts and the tailplane of the craft.
The systems of flight control type are also complex and expensive and rely on the possibility offered to them of acting on the control surfaces of the craft. Not all helicopters may be so equipped. Moreover these systems, through their corrective action, do not allow the crew to anticipate the situation. They likewise do not make it possible to protect the lateral parts or the front of the craft and they offer reliable protection only in relation to the ground or obstacles situated under the craft. They do not protect the craft in relation to mobile obstacles.